1. Field of the Invention
The present invention relates to current detector units for detecting a current and to motor control devices that drive and control a motor.
2. Description of Related Art
In order to perform vector control of a motor by supplying three-phase alternating-current power to the motor, it is necessary to detect two phase currents (for example, a U-phase current and a V-phase current) among three phase currents consisting of U-phase, V-phase, and W-phase currents. In general, detection of two phase currents is performed by using two current sensors (such as current transformers). However, the use of two current sensors undesirably increases overall cost of a system incorporating the motor.
It is for this reason that there has conventionally been proposed a technique by which a bus current (direct current) between an inverter and a direct-current power supply is detected with a single current sensor, and, based on the bus current thus detected, two phase currents are detected. This technique is called a single shunt current detection technique.
FIG. 19 is an overall block diagram showing a conventional motor drive system employing the single shunt current detection technique. An inverter (PWM inverter) 902 includes three half-bridge circuits provided one for each of three phases and each having an upper arm and a lower arm. The inverter 902 performs individual switching of each arm in accordance with specified three-phase voltage values fed from a controller 903, and thereby converts a direct-current voltage from a direct-current power supply 904 into a three-phase alternating-current voltage. The three-phase alternating-current voltage thus obtained is supplied to a three-phase permanent-magnet synchronous motor 901 to drive and control the motor 901.
A line connecting each lower arm provided in the inverter 902 with the direct-current power supply 904 is called a bus 913. A current sensor 905 transmits, to the controller 903, a signal indicating a bus current flowing through the bus 913. The controller 903 performs sampling of an output signal of the current sensor 905 with appropriate timing, and thereby detects a phase current of a phase with a maximum voltage level (maximum phase) and a phase current of a phase with a minimum voltage level (minimum phase), namely, two phase currents. That is, upon detection of two phase currents, a three-phase current of the inverter 902, namely a motor current, is automatically obtained.
If there is a sufficient level difference among the voltage levels of different phases, two phase currents can be detected in the above-described manner. However, if the voltage levels of the maximum phase and intermediate phase come close to each other, or the voltage levels of the minimum phase and intermediate phase come close to each other, it becomes impossible to detect two phase currents. Incidentally, the single shunt current detection technique will be described later in an embodiment of the present invention, along with the reason why it becomes impossible to detect two phase currents.
In view of this, there has been a conventional method in which the single shunt current detection technique is performed such that, if a difference between any two phase voltages is relatively large, a present motor current is detected by using present bus current information; if the difference is relatively small, a present motor current is estimated by using the bus current information detected in the past.
In this conventional method, a voltage threshold value is previously set, and the voltage threshold value thus set is compared with the difference between two phase voltages to make a judgment whether the difference is relatively large or small. However, with this conventional method, since the voltage threshold value has to be set with allowance made for various factors, there is a possibility that, although in actuality two phase currents can be detected, it is judged that they cannot be detected.